1. Field of the Invention
The invention relates to the preparation of optionally substituted diphenylamine by reaction of optionally substituted aniline with optionally substituted cyclohexanone using Rh catalysts from halogen-free salts.
2. Description of the Related Art
Supported Rh catalysts for the preparation of optionally substituted diphenylamine by reaction of optionally substituted aniline with optionally substituted cyclohexanones are known, for example, from EP 535 484.
For this purpose, EP 535 484 describes a supported catalyst containing rhodium or a combination of rhodium with another platinum metal from the group of palladium, platinum, ruthenium or iridium. The noble metals are present in a total amount of from 0.05 to 5% by weight, based on the total weight of the catalyst. The catalyst described in EP 535 484 additionally contains from 1 to 12% by weight of alkali metal and optionally from 1 to 12% by weight of sulphur and also optionally from 0.05 to 8% by weight of chromium and manganese, based on the support material.
The Rh catalyst described in EP 535 484 is prepared, after the heat treatment of the catalyst support treated with chromium and manganese, by applying rhodium or rhodium and a further noble metals by known methods. A description is given of the deposition of the rhodium or the rhodium and the further noble metals from the aqueous solution of their salts, with salts such as chloride, nitrate, acetate being mentioned without any differentiation or preference. However, the examples in EP 535 484 are based only on the use of chlorides.
The deposition of the Rh salt and optionally the noble metal salts is carried out by precipitation with basic ammonium or alkali metal compounds in aqueous solution, which basic compounds are applied either before or after the Rh or the noble metals. Independently of the treatment with sulphur compounds, which is optionally carried out in a further preparation step, no washing procedure is finally carried out prior to drying, reductive pretreatment and the use of the catalyst for the preparation of the optionally substituted diphenylamines. The anion of the Rh salt or the noble metal salts, in the example the chloride, thus remains in the catalyst.
In the preparation, for example, of the unsubstituted diphenylamine from aniline and cyclohexanone over the Rh catalysts from the chlorides described in the examples of EP 535 484, a series of byproducts including N-cyclohexylaniline, carbazole, cyclohexanol, phenol and benzene are formed. In addition, the intermediate of the reaction to give diphenylamine, namely N-cyclohexylideneaniline, is obtained.
The amounts of the byproducts and intermediates formed are strongly dependent, according to our own studies, on the pretreatment conditions of the catalyst (reduction temperature and time). EP 535 484 recommends a reduction temperature of from 120.degree. to 400.degree. C. In EP 208 923, in which the catalyst of EP 535 484 is claimed, the reduction temperature specified is even the further range of from 120.degree. to 450.degree. C. and the duration specified is from 30 to 80 hours.
Furthermore, the composition of the product stream of the reaction alters greatly during the running-in of the catalyst. However, although the selectivity for the desired product diphenylamine increases during this time, a considerable amount of byproducts which can mostly no longer be used in the process is obtained depending on the initial selectivity and depending on the required duration of the running-in phase; in particular, benzene interferes as byproduct. Long running-up phases with gradually changing product composition likewise have an adverse effect on the work-up of the product stream. Furthermore, the strong dependence of the product composition on the duration and temperature of the hydrogenative pretreatment of the catalyst places high demands on the exact adherence to these conditions.
It would therefore be desirable to find an Rh catalyst for preparing optionally substituted diphenylamine from optionally substituted aniline and optionally substituted cyclohexanone which avoids the difficulties described: desirable aspects are low sensitivity of the product composition to the type of reductive pretreatment of the catalyst, high initial selectivity and short running-in time of the catalyst into its steady state of maximum selectivity with simultaneously minimal benzene values.